What do you want your PC for ?
Build an office or Kids PC .
Mainly used for homework and games.
This would most likely entail a smaller budget and have to take into consideration the software required by the child's school.
Most likely Windows with Office or such.
What do you want your PC for ?
Build a cheap, general use PC.
Mainly for browsing internet, sending email etc.
This would involve shopping for bargains and using free software as much as possible.
Maybe even using a Raspberry Pi.
What do you want your PC for ?
Build a gaming PC.
This would entail looking at the specifications of the games required and shopping around for components
that meet the specs and will have a reasonable shelf-life for new and upcoming games in order
to get the best value for what could be a sizeable expenditure.
The Processor (CPU)
The Motherboard
The Case
The Ram
The Graphics Cards
The Hard Drive
The Optical drive (CD / DVD drive)
The Power Supply
WiFi Card
SD Card Reader
The brain of computer
Performs the basic operations
The base component that connects other components
Contains most of the machine features (USB ports, sounds video & WiFi cards slot, ..)
The computer's short term memory
The link to your screen
Hard or solid state drives, are long term data storage components
To read and write CDs, DVDs and Blu-Rays
The last component to shop for, once the other components are selected
WiFi card, SD and other memory support adapter